Image setters are pre-press devices that are used to expose a photosensitive medium based on an image to be printed. Generally, in the past, most image setters have had an internal-drum configuration, in which the photosensitive medium is laid on the inner surface of a cylindrical drum. A laser beam, typically originating from the drum's axis, is then raster scanned across the photosensitive media to selectively expose it to form the image. External drum image setters, however, are becoming increasingly common.
Although digital photosensitive media, for example, are optically sensitive, the laser beam power from the scanning laser is relatively high in image setters deployed in industrial environments. This characteristic is due to the fact that media throughput is one of the main factors in determining the productivity, and thus value, of an image setter. Consequently, the speed at which the beam is scanned across the photosensitive media is typically maximized. As a result, since the time the laser beam is exposing any given region of the photosensitive media is reduced, the beam's power must be increased to deposit enough energy in the media to initiate the necessary chemical reactions for exposure.
In order to achieve the necessary beam power, many times solid-state lasers, such as Nd:YAG lasers, are used. These lasers can not be switched on and off at the necessary speed required to form the image. Consequently, the lasers are typically run in a CW mode, and a separate laser beam intensity modulation system is used.
Commonly, at the core of the laser beam intensity modulation systems are devices called acousto-optic modulators (AOM's). These are beam steering devices, which, in an unactivated state transmit the beam directly through the crystal substrate, referred to as the zero-order beam or light, and when activated by a radio-frequency source, refract the beam at known angle, referred to as the first-order beam or light.
AOM's are relatively efficient devices. When unactivated, they transmit most of the light input to the crystal through as the zero-order light, except for some incidental scattering. When activated, most of the beam, approximately 80% or more, is diverted or steered to emerge as a first-order beam, while 20% is still transmitted through the crystal as zero-order light.
In most image setters using AOM's in the laser beam intensity modulation system, the first-order light from the AOM is used to expose the photosensitive media. In other words, when a given region of the photosensitive media is to be exposed, the AOM is activated to steer the laser beam into an optical path that leads to the photosensitive media. In an unactivated state, the zero-order light passing through the AOM does not reach the photosensitive media, being dumped outside the optical path.
The use of the first-order light to expose the photosensitive medium works well as a strategy for meeting the exposure requirements of common digital media. The first order beam provides a very high dynamic range between the light levels of the first-order beam and the relatively insignificant scattering that is transmitted along the optical path when the AOM is unactivated. As a result, there is little clouding of the medium due to incidental exposure of those areas of the photosensitive medium that are intended to not be exposed based upon the image.